1. Field of the Invention
The present invention relates to an optical scanning apparatus which is used for image forming apparatuses such as a laser printer and a laser facsimile.
2. Description of Related Art
An optical scanning apparatus (scanner unit) used for an image forming apparatus such as a laser beam printer deflects and emits a light beam, which corresponds to image information and is emitted from a semiconductor laser, with a polygonal rotatable mirror which performs a high-speed rotation. An electrostatic latent image according to the image information is formed on a photosensitive member by scanning a photosensitive member by the light beam emitted from the optical scanning apparatus. Subsequently, the electrostatic latent image on the photosensitive member is visualized in a toner image by a developing apparatus, and this is transferred to a recording medium such as recording paper. The recording medium is sent to a fixing apparatus after that, and printing is completed by performing the heat fixing of the toner on the recording medium.
In recent years, acceleration of the image forming apparatus progresses, and in order to correspond to this acceleration, also in an optical scanning apparatus, the rotating speed of a polygonal rotatable mirror arrives at several tens of thousands of rpm increasingly.
By the way, a mounting structure to the scanner unit of a polygonal rotatable mirror drive motor which is provided in a scanner unit, is common as the structure disclosed in Japanese Patent Application Laid-Open No. 2003-295099. The structure is shown in FIG. 9.
As shown in FIG. 9, a polygonal rotatable mirror 22 is put on a flange 23 installed on a rotary shaft 20, and is fixed by a leaf spring 21. The rotary shaft 20 is inserted rotatably into bearing sleeve 31, and an end of the rotary shaft 20 contacts with a thrust plate 32 to constitute a rotor 24. The rotor 24 is arranged so as to face a stator coil 26. The motor in this embodiment has the dynamic pressure liquid bearing structure that the dynamic pressure generating groove 25 is formed in the rotary shaft 20 and oil is contained in the sleeve 31. When a current flows into the stator coil 26, the rotor 24 rotates, and thereby, the polygonal rotatable mirror 22 also rotates.
A motor drive circuit is mounted on a printed board 27 which is made of metal. This printed board 27 is fixed to a printed board mounting surface 30 provided on the outer periphery of the bearing sleeve 31 by a joint method such as caulking. Hence, the printed board 27 and bearing sleeve 31 can be dealt as one piece. In addition, the printed board 27 is fixed to a mounting surface 28 of an optical box 29, which contains a lens, not shown, the above-mentioned drive motor, and the like, with fixing means such as screws. At this time, the bearing sleeve 31 is engaged in a hole, provided in the optical box 29, with almost no gap.
This structure has a merit that the drive motor for the rotation of the polygonal rotatable mirror 22 is simply mounted in the optical box 29 of a scanner unit, and is effective for the cost reduction of the scanner unit.
Nevertheless, when an optical scanning apparatus with such structure is mounted in a high-speed printer with a large number of output sheets per unit time, the following subjects should be taken into consideration. In order to make an optical scanning apparatus correspond to high-speed printing, there are a method of increasing the number of faces of a polygonal rotatable mirror, a method of increasing a number of light beams, a method of increasing the rotational speed of a polygonal rotatable mirror, and the like. Among these, when setting the rotational speed of a polygonal rotatable mirror highly, there arises a problem that the temperature of a bearing of the polygonal rotatable mirror drive motor rises and the durability of the motor falls. In particular, when the rotational speed of a polygonal rotatable mirror is high, the loss in a bearing also becomes large, and a calorific quantity is also large, and hence, this problem becomes still more remarkable.
What is conceivable so as to solve the above-mentioned problems is means of mounting a metal heat release member in a bearing as described in, for example, Japanese Patent Application Laid-Open H5-303050 and Japanese Patent Application Laid-Open H6-59208. Nevertheless, this causes another problem that a part count increases and the component cost of an optical scanning apparatus increases.